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> Hi, I'm having a bit of a slow day... I'm trying to synchronise two reads
> from two channels and can't get my fuzzy head round the problem.
>
>
Go doesn't have support for an (atomic) read from multiple channels at
once. There are other systems which can do this (JoCaml, join-calculus,
Concurrent ML, Haskell's STM, Reagents[0] in Scala, ...)
However, as you found out, one can often rearrange the code base as to
avoid the problem. There is also the problem of more advanced
select-constructs not being free in the performance sense: atomic sync on
multiple channels tend to be rather expensive.
One reason it isn't that simple to implement is to consider the following
(hypothetical) snippet:
select {
case w := <-available && urlChan <- value:
..BODY..
}
}
in which the atomic operation must simultaneously receive a message on the
'available' channel and send a message on the 'urlChan' channel. And
further, since you introduced && to mean logical-and-between-channels, you
might as well introduce || to mean logical-or-between-channels. Once there,
you may want to introduce a new type
event t
for some type t alongside 'chan t'. Hence the expression (<-available)
becomes an expression of type 'event *worker' and now they can be composed
via && and ||. This means we can get rid of select since it is just an
expression which successively applies || to a slice of values. If we then
introduce an new statement, sync, which turns 'event t' into a 't', we are
done. And we have almost implemented the basis of Concurrent ML in the
process.
In short, you could be opening a regular can of worms if you start allowing
for more complicated select-expressions. Hence the school of keeping them
relatively simple.
[0] https://people.mpi-sws.org/~turon/reagents.pdf
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